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develops at f1.25 mJ/cm 2 (the lasing threshold is clearly seen in the pumpdependentmode intensity in the inset in Fig. 26b). The position of this mode(612.0 nm) corresponds to the optical-gain maximum (solid circles in Fig.26b). As the pump fluence is increased, additional WG modes develop on thelow-energy side of the 612.0-nm mode, which is consistent with an asymmetricshape of the gain band. From the intermode spacing (f0.75 nm), one canderive an effective index of the gain medium of 1.8, indicating that WG modesare confined primarily within the NQD layer.In Ref. 63, NQD lasing structures were fabricated by combining solgeltitania waveguides [11] containing large NQD volume fractions withDFB gratings (Fig. 27a, inset). The gratings were fabricated by patterningthermal oxide layers on silicon substrates. The structures were completed byspin-coating thin NQD/titania films on the top of the gratings.In the DFB/NQD structure, the refractive index is spatially modulateddue to the thickness modulation of the nanocrystal/titania film caused by theunderlying grating. This modulation leads to the formation of the stop bandwith a width of f10 nm. Above the lasing threshold, a sharp mode at theedge of the stop band grows superlinearly as the pump power is increased (seespectrum in Fig. 27a). The emission from the front of the structure collapsesinto a visible laser beam, and the luminescence spot on the film tightens into abright narrow line indicating a transition to the lasing regime. The DFBstructures were used to demonstrate lasing with a tunable emission colorusing NQDs of different sizes, as illustrated in Fig. 27b. This figure showssharp lasing lines (solid lines) that are compared to spontaneous emissionspectra (dashed lines) recorded in the ‘‘subthreshold’’ regime. The lasingspectrum was tunable from 621 to 560 nm by changing the NQD radii from2.7 to 1.7 nm.VI.CONCLUSIONS AND OUTLOOKOptical gain and lasing in strongly confined, colloidal quantum dots is a new,exciting area in NQD research. Due to very small sizes and the strongFigure 27 (a) The room-temperature laser spectrum of a DFB device fabricatedusing core-shell CdSe/ZnS NQDs with a 2.5-nm CdSe core radius. Inset: the scanningelectron microphotograph of a typical DFB device used in Ref. 63. (b) Spontaneousemission spectra (dashed lines) of CdSe NQDs with radii 2.7, 2.4, 2.1, and 1.7 nm (sizedecreases from left to right) in comparison with lasing spectra (solid lines) of thesame samples incorporated into a DFB device. The lasing emission occurs at 621 nm(R = 2.7 nm), 607 nm (R = 2.4 nm), 583 nm (R = 2.1 nm), and 560 nm (R = 1.7 nm).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
coupling between ‘‘volume’’ and interface states, optical-gain properties ofNQDs are significantly different from those of well-studied semiconductornanostructures fabricated by epitaxial techniques. The specificity of colloidalnanosystems results in unexpected complications for the development ofoptical gain and stimulated emission such as highly efficient, nonradiativeAuger recombination, and strong photoinduced absorption associated withcarriers trapped at NQD interfaces. On the other hand, NQDs offer numerousadvantages for applications as optical-gain media. These advantages are dueto the wide-range tunability of emission color provided by the facile manipulationof NQD sizes, large separations between energy states, and theflexibility of the chemical syntheses used for the NQD fabrication. Usingappropriate materials, one can fabricate strongly confined NQDs emitting inspectral ranges from the far-infrared to ultraviolet. Furthermore, NQDs canbe easily incorporated into transparent host matrices and different photonicstructures including microcavities and photonic crystals.The first demonstrations of NQD lasing devices operating with a widerange of colors indicate a high technological potential of NQD materials asnew types of tunable lasing medium. On the other hand, this work also opensnew and interesting avenues in fundamental research, specifically in the arenaof light–matter interactions in photonic structures. Tunable electronic structuresin NQDs, combined, for example, with a tunable density of photonicstates in high finesse microcavities, can provide a wide range of control overelectron–photon coupling and allow, in principle, a realization of a ‘‘strongcoupling’’regime between NQDs and cavity modes. Such a regime is requiredfor single-dot lasing and quantum information processing using, for example,NQD spins coupled through a microcavity mode.There are currently several important challenges in NQD lasing research.Some of them are associated with materials quality issues and someare of conceptual character. On the materials’ side, further improvements inmonodispersity and surface passivation are required to improve NQD lasingperformance. There is also a need for new, high-quality NQD materials forthe extension of operational wavelengths into infrared and ultraviolet spectralranges. Recent demonstration of high-quality PbSe NQDs [65] is promisingfor realizing lasing in the near-infrared spectral range, specifically in therange of communication wavelengths. Further work on reliable proceduresfor incorporating NQDs into solid-state matrices is an important step towarddurable and stable NQD materials for lasing applications.An important conceptual challenge focuses on carrier electrical injection.Previous approaches to electrical pumping of NQDs were based oncombining NQDs with conducting polymers in hybrid organic–inorganicstructures [66,67]. The performance of these devices was severely limitedbecause of low carrier mobilities in both polymer and NQD device compo-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Copyright 2004 by Marcel Dekker, In
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This book covers several topics of
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esult, some exciting topics were no
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3. Fine Structure and Polarization
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9. III-V Quantum Dots and Quantum D
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ContributorsUri BaninThe Hebrew Uni
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1‘‘Soft’’ Chemical Synthesi
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structure of energy states leads to
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growth can proceed by Ostwald ripen
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Figure 3 Transmission electron micr
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Figure 4 Temporal evolution of the
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No. 26, are f85% (Fig. 6) [21]. Alt
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tion spectra and broad PL spectra.
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ing surface-to-volume ratio with di
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Figure 8 Photoluminescence spectra
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lattice mismatch. Such a large latt
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match between InAs and ZnS of f11%.
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successfully repeated for up to thr
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Under a different growth regime, on
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Figure 14 Atomic model of the CdSe
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‘‘Teardrop-shaped’’ particl
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Figure 17 High-resolution TEMs of C
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allows isolation of tetrapods in f8
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ligand concentrations yield a reduc
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een determined and quantitatively c
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tion volumes were also shown to be
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synthesis temperatures of z400jC ar
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Figure 23 (a) Photoluminescence spe
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levels (fV1 Mn per NQD). Despite in
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Figure 25 X-ray diffraction pattern
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Figure 27 Transmission electron mic
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An additional factor that strongly
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Figure 30 (a,b) Schematics illustra
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Slow, controlled precipitation of h
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Figure 34 Schematic illustrating th
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achieving biological compatibility
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47. Yu H.; Gibbons P.C.; Kelton K.F
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2Electronic Structure inSemiconduct
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Figure 2 (a) Simple model of a nano
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electron and hole to be treated as
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independently, Eq. (13) is commonly
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a better description of the bulk ba
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investigated. For optical experimen
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Figure 4 (a) Absorption (solid line
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Figure 6 Normalized PLE scans for s
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Figure 8 A simplistic model for des
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Figure 10 Theoretically predicted p
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Figure 12 Schematics depicting the
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Figure 14 Calculated band-edge exci
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Figure 15 Absorption (solid line) a
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Figure 18 (a) Calculated band-edge
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IV.BEYOND CdSeA. Indium Arsenide Na
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the six-band Luttinger Hamiltonian.
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11. Norris, D.J.; Efros, Al.L.; Ros
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69. Gaponenko, S.V.; Woggon, U.; Sa
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formation of a long-lived dark exci
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where the constant A is determined
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In crystals for which the function
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The respective wave functions areC
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passive, as was shown in Ref. 12. T
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square of the matrix element of the
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where e F = e F ieV and e F F = e x
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see that for all nanocrystal shapes
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optical recombination of the excito
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B. Recombination of the Dark Excito
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where x = cos h and f = l B g e H/3
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The theory of the polarization memo
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Figure 7 The size dependence of the
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state would have an infinite lifeti
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crystal axis [see Eq. (40)]. As a r
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time of the exciton momentum relaxa
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One must also account for the influ
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observed in one of the first studie
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REFERENCES1. Bawendi, M.G.; Wilson,
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4Intraband Spectroscopyand Dynamics
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The solid line in Fig. 1 shows the
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Figure 2 FTIR spectra of n-type CdS
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of the center frequency. The experi
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limit given by radiative relaxation
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from a long lifetime due to the pho
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are two natural approaches to study
Page 173 and 174: 32. Inoshita, T.; Sakaki, H. Physic
Page 175 and 176: continuous spectral tunability over
Page 177 and 178: function and envelope function mome
Page 179 and 180: ottleneck’’ [14,28]. Further re
Page 181 and 182: in NQDs is dominated by nonphonon e
Page 183 and 184: Figure 4 Dynamics of the IR postpum
Page 185 and 186: Figure 5 (a) Time-resolved PL spect
Page 187 and 188: Figure 6 (a) The time delay of the
Page 189 and 190: and due to Auger-type e-h interacti
Page 191 and 192: Figure 9 Dynamics of the 1S bleachi
Page 193 and 194: significantly greater than the fast
Page 195 and 196: Figure 11 (a) Pump-intensity-depend
Page 197 and 198: NQD size. For small NQD sizes (R =
Page 199 and 200: Figure 14 Nonlinear absorption/gain
Page 201 and 202: sorption change associated with a s
Page 203 and 204: where n h em is the hole ‘‘emit
Page 205 and 206: ultrafast (subpicosecond to picosec
Page 207 and 208: Figure 18 Schematic of transitions
Page 209 and 210: Figure 19 Dynamics of pump-induced
Page 211 and 212: where n i (i=1, 2 . . . , N ) is th
Page 213 and 214: Figure 21 (a) Two-e-h-pair (biexcit
Page 215 and 216: the volume fraction (filling factor
Page 217 and 218: intensity dependence of this peak (
Page 219 and 220: Copyright 2004 by Marcel Dekker, In
Page 221 and 222: can contribute to the saturation of
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Page 227 and 228: numerous discussions on the photoph
Page 229 and 230: 53. Kang, K.; Kepner, A.; Gaponenko
Page 231 and 232: the ‘‘on-off ’’ emission in
Page 233 and 234: parallel form of data acquisition i
Page 235 and 236: Figure 3 (a) Spectral time trace of
Page 237 and 238: transition energies. In fact, these
Page 239 and 240: distribution (dark line) does not d
Page 241 and 242: exposure to only room light. In our
Page 243 and 244: statistics for the off times are in
Page 245 and 246: 230Shimizu and BawendiCopyright 200
Page 247 and 248: Figure 10 (a) Time trace of a CdSe(
Page 249 and 250: and excited QD core states to fluct
Page 251 and 252: arrows indicate the on-time truncat
Page 253 and 254: W. K. Woo and V. C. Sundar for assi
Page 255 and 256: size allows the electron affinity a
Page 257 and 258: II.THEORY OF ELECTRON TRANSFER BETW
Page 259 and 260: For the specific case of charge tra
Page 261 and 262: dominates, the mobility is often fi
Page 263 and 264: where the constant A and the temper
Page 265 and 266: components of modulation which are
Page 267 and 268: nanocrystals [41], understanding ph
Page 269 and 270: and ionization potential through tw
Page 271 and 272: quantum dots. Furthermore, because
Page 273 and 274: For many applications, a host mater
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form blends with morphologies that
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Figure 11 Photoluminescence efficie
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Figure 12 (a) Room-temperature PIA
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discussed briefly in Section IV. Ch
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dithiolates to thiol-terminated DNA
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Films of passivated CdSe nanocrysta
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Figure 16 Photocurrent action spect
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decay with stretched exponential ki
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siderably larger than might be esti
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dispersing CdSe nanocrystal chromop
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memory and charge storage effects [
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all) of the optically excited elect
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composites of nanocrystals and conj
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55. Asbury, J.B.; Hao, E.C.; Wang,
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108. Morgan, N.Y.; Leatherdale, C.A
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The approaches to fabrication of se
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Figure 1 Experimental realization o
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Due to this voltage division, the m
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Figure 3 Simulated tunneling spectr
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electron charging. In both positive
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Figure 5 shows the typical features
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Figure 7 Map of levels for InAs nan
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Figure 8 Scanning electron microsco
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D CB = 0.31 eV is thus obtained. On
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Figure 11 Correlation of optical an
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atomistic approach based on pseudop
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charging indicated that the tunneli
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capacitance values were also kept t
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could not be detected in the QD/DT/
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Figure 18 Tunneling conductanceF sp
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data in the inset of Fig. 19 repres
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corresponding to the s-like wave fu
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7. Grabert, H.; Devoret, M.H., Eds.
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66. Su, B.; Goldman, V.J.; Cunningh
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dimensional confinement are created
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Figure 1 Transmission electron micr
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The room-temperature absorption and
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narrower in samples with larger mea
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GaInP 2 QDs from a plot of the squa
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crystal, indicating lattice-matched
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Figure 5 Evolution of Stranski-Kras
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C. Efficient Anti-Stokes Photolumin
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Because HF treatment has been shown
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intensity of the PL when it is on a
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eV stems from recombining carriers
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Figure 16 Model to explain two-colo
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although this term is not rigorousl
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10 ps (about an order of magnitude
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electron relaxation is inhibited an
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QDs, the nature of the QD capping s
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QD solution. For an interdot distan
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emission spectra of the two individ
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Figure 23 Change of the PL intensit
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(viz. the absorbed light intensity)
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Figure 25Impact ionization in QDs.m
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prevent electron-hole recombination
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36. Miller, R.D.J.; McLendon, G.; N
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91. Vurgaftman, I.; Singh, J. Appl.
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139. Mićić, O.I.; Ahrenkiel, S.P.
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10Synthesis and Fabrication of Meta
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Figure 2 (A-C) Progression of HR-TE
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Figure 3 Schematic for gold nanocry
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Nanocrystal growth can occur by two
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on the other hand, provide an ensem
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Figure 6 (a) SAXS patterns for disp
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Figure 8 The gold nanocrystal film
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of the stabilizing ligand, and the
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successfully modeled the 2D island
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2. Steric Stabilization and a Soft
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are fully extended. Moving away fro
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Figure 11 High-resolution SEM image
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Figure 13 (A) Transmission electron
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function of the density of localize
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Figure 15 High-resolution SEM image
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thiol-capped nanocrystals [2]. The
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41. Ackerson, B.J. Nature 1993, 365
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with the effect of the particle com
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Figure 1a shows the surface plasmon
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Figure 2 (a) Ultraviolet-visible ab
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agents [34]. The short-wavelength b
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Figure 4 (a) Plot of the plasmon ab
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the framework of traditional Mie’
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show that effects due to the surrou
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is located at the position of the g
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Figure 8 Ultraviolet-visible absorp
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laser pulses while being mixed in t
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Figure 10 shows HR-TEM images of go
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final irradiation product. At the s
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following the laser excitation appe
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36. Papavassiliou, G.C. Prog. Solid
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particles to expand. Because the he
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was frequency doubled in a 1-mm B-
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Figure 2 Frequency of the acoustic
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Figure 3 Change in radius (DR/R) ve
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In this model, the electrons couple
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Figure 5 Transient bleach data for
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Figure 7 (a) Transient bleach data
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that the particles with >80% Au hav
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3. Del Fatti, N.; Valle´e, F.; Fly
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